Some black holes are 'closet eaters'

Black holes that are devouring their surroundings are among the brightest objects in the universe, shining like beacons from billions of light years away. But astronomers have found a strange new class of these objects that behave completely differently - 'closet eaters' that emit virtually no detectable radiation as they wolf down nearby matter.

The research may shed light on why the colossal black holes at the centres of some galaxies are gluttons, while others, such as the one inside the Milky Way, fast most of the time.

Until now, the enthusiastic eaters - known as active galactic nuclei, or AGN - were all thought to share the same essential structure. In this 'unified model', a doughnut-like disc of gas and dust, or torus, surrounds the supermassive black hole.

The AGN shine so brightly because matter from the torus is drawn towards the black hole, emitting radiation as it heats up and its magnetic fields twist and reconnect. Any differences in the nature of the radiation astronomers observe from the objects have been attributed to the angle at which they were viewed (see illustration below right).

Now, about eight AGN have been found that do not fall into this unified model. They were initially discovered using the Burst Alert Telescope on NASA's Swift space observatory, which observes high-energy X-rays. Follow-up observations with Japan's Suzaku satellite, which detects a wider range of X-rays, then confirmed that the objects did not radiate X-rays at lower energies.

'Filled doughnut'

Only AGN can emit X-rays at the energies Swift observed, suggesting the new objects are indeed ravenous black holes. But the fact that Suzaku did not detect them at lower energies suggests they are completely surrounded by gas and dust - which absorbs lower-energy X-rays - rather than a relatively flat, dusty torus.

"We're finding objects that don't have the shape of a doughnut . . . that don't have this hole in the middle," says team member Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US. "The dust and gas is in a big mish-mosh in the centre. This is unexpected."

Jack Tueller, another team member at Goddard, suggests several explanations for this structure. The black hole may in fact be surrounded by a torus, but the torus-black hole combination may be embedded in a huge cloud of dust and gas that absorbs most wavelengths of light.

Another possibility is that the particles of gas and dust surrounding the black hole are heated in such a way that they have random velocities, producing a very thick disc of material that does not have a hole in its centre. "It's like a filled doughnut," Tueller told New Scientist.

High-energy glow

The discoveries of these black holes - which have evaded detection till now because they do not radiate at most wavelengths of light - suggest astronomers have underestimated the number of AGN in the universe by perhaps 20%, says Mushotzky. This could help astronomers better account for the source of diffuse, energetic radiation that pervades the universe, called the cosmic high-energy background, agrees Tueller.

"Another big mystery we don't understand is why are some black holes radiating and others not?" says Mushotzky. "If you don't observe all the objects that are radiating - and we are finding hidden ones - we can't test these ideas out properly."

Current theories suggest mergers between galaxies push gas into their cores, igniting the black holes there as AGN. "If that idea is correct, as we do our survey, we should find that many of the objects we're detecting also have the signature of mergers about them - they'll have either close companions, or be highly distorted, or have tidal tails," he says.

The team hopes to get clues about the structure of the objects by observing their spectra with the Spitzer Space Telescope, which detects infrared light. The dust and gas around them should absorb the objects' high-energy radiation and re-emit it at infrared wavelengths that would be more energetic in warmer regions close to the black hole and less energetic at greater distances.

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Newly discovered 'active galactic nuclei' appear to be surrounded by a cloud of gas and dust that blocks most wavelengths of light from escaping (Illustration: Aurore Simonnet/Sonoma State University)